It sounds like something out of a comic book, or Lord of the Rings, I know. But there's a chance that invisibility real-life, honest-to-God invisibility may actually be possible, some day.
The technology doesnt come from some dubious unknown inventor, but from Professor Sir John Pendry, the legendary theoretical physicist, I write in this months BBC Focus magazine the world's best science and technology monthly. Pendry has developed the concept of metamaterials, which have properties determined by their structure rather than their composition. This can give them 'impossible' properties, such as a negative refractive index. Initially, there was some debate about whether this could ever be achieved. But the proof came last year with the demonstration of a superlens capable of beating any lens made of normal material. 
By utilizing metamaterials, it should be possible, in theory, to create what Pendry calls an invisibility cloak -- although invisibility shell might be more accurate as it will need to be rigid. Such a cloak would divert any incident light around its surface and release it on the same path on the other side: to any observer the wearer is invisible.
Interestingly, Pendrys work on metamaterials started when he was working for Marconi. He was looking at the application of carbon fiber for a stealth coating when he realized that its interaction with radar was determined by the length of the fibers it was effectively acting as an array of tiny aerials and that the same effect could have many other applications.
Invisibility in the optical spectrum will be challenging because metamaterials will need to be constructed on a scale corresponding to the wavelengths of visible light, which is just a few hundred nanometres. That technology will not be around for at least five years.
But radar invisibility is much easier because radar wavelengths are in the centimeter range. Pendrys colleague, Dr. David R. Smith at Duke University, is already working on a microwave metamaterial. Results are expected within eighteen months.
Unlike existing stealth techniques, a metamaterial should in principle be able to make an aircraft (or missile) literally invisible to any radar from any aspect.
There are likely to be other metamaterials along later. As Pendry explained, they can have all sorts of mechanical or acoustic properties as well as affecting light or other radiation. But for the mean time, we are likely to have out hands full just thinking of applications for invisibility.
The biggest question is likely to be the width of spectrum that any given material can handle. According to Pendry, a sufficiently deep metamaterial should be able to cope with a very wide spectrum. This might include all visible wavelengths plus a chunk of infra-red and UV. However, its clear that even very limited invisibility could be a major military asset.
-- David Hambling
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